1. Field of the Invention
The present invention relates to an image processing apparatus and image processing method for generating image data to be supplied to an image display apparatus which inverts the drive polarity of display elements in each of plural sub-frames formed from a frame of a moving picture to display the moving picture.
2. Description of the Related Art
A cathode ray tube (CRT) has been synonymous with a moving picture display device for a television and the like. However, liquid crystal displays (LCDs), plasma displays (PDPs), and field emission displays (FEDs) have been put into practical use in recent years. There are displays of various display types now. The displays of these types adopt different display methods. For example, display devices based on a liquid crystal display (e.g., a direct viewing type LCD, liquid crystal rear projector, and liquid crystal front projector) use many scanning methods. In any case, the period of outputting light in each pixel portion occupies a large part of one frame. For this reason, such a display is called a hold-type display.
On the other hand, in a CRT or FED, light is output in each pixel portion once in a frame. The period of emitting light is much shorter than the frame length and is normally 2 msec or less. For this reason, such a display is called an impulse-type display.
There also exist a PDP and field sequential display (FSD) which are of types different from the above-mentioned classes.
The hold-type display and impulse-type display have the following features.
The hold-type display: A display of this type emits light during a large part of the frame period. Hence, temporal variations of light intensity are small, and flicker is rarely observed. However, pursuit (pursuing a moving portion in a moving picture by eyes) makes the motion blur relatively large in accordance with the length of the period of emitting light in a frame. This motion blur here is different from that caused by the response characteristic of a display device.
The impulse-type display: A display of this type has a very short period of emitting light in a frame. Hence, temporal variations of light intensity are large, and flicker synchronous with a frame is observed. However, the motion blur in pursuit is rarely observed. It is, therefore, possible to obtain a resolution almost equal to that of a still object.
In general, the period of emitting light in a display changes depending on the display method and display device. The hold-type display and impulse-type display are poles apart in terms of in the period of emitting light. The period of emitting light and the degree of motion blur are almost proportional to each other. Longer periods of light emission result in larger amounts of motion blur. Shorter periods of light emission result in smaller amounts of motion blur. On the other hand, concerning flicker synchronous with a frame, longer periods of light emission result in smaller flicker. Shorter periods of light emission result in larger flicker. In this manner, the motion blur and flicker have trade-off relationships with respect to the period of emitting light.
A solution to the motion blur and flicker is multiplying the frame frequency by N. In many cases, N=2. That is, the frame frequency is doubled. When the frame frequency is doubled, the period of emitting light in each double speed-frame is halved. This also almost halves the motion blur. Regarding flicker as well, if an initial frame frequency of 60 Hz is doubled to 120 Hz, the frame frequency falls outside the response characteristic of human eyes. Hence, no flicker is observed.
As described above, multiplying the frame frequency by N has a large effect but poses a new problem.
For example, when the frame frequency of an original picture signal is 60 Hz, the picture information is updated every 1/60 sec. If the frame frequency is doubled to display a picture at 120 Hz, necessary picture information is missing every other frame. As a measure, identical pictures are displayed, for example, twice if the frame frequency is doubled. This solves flicker but cannot improve the motion blur in the original picture. In an impulse-type display, double pictures are observed by pursuit.
To double the frame frequency, two methods are mainly used.
The first method detects the motion of an object in an original picture and predicts pictures between two frames. This is generally called an “intermediate picture insertion method based on motion compensation”. In the first method, the amount of calculation is enormous, and a prediction error occurs under a specific condition.
In the second method, first, filtering is performed for each frame of an input picture to separate the spatial frequency into a high frequency component which greatly concerns the motion blur and a low frequency component which greatly concerns flicker. Then, the high frequency component is concentrated to one sub-frame (one of two double speed-frames corresponding to the original frame). The low frequency component is distributed to both sub-frames (both of the two double speed-frames corresponding to the original frame). The second method will be called a “sub-frame display method based on spatial frequency separation”.
In a display using a liquid crystal panel, if the liquid crystal panel is driven while the DC balance is disturbed, electrical charges are accumulated between electrodes to degrade the image quality. To avoid this, the drive polarity of the liquid crystal panel is inverted every predetermined cycle to cancel the DC offset of the voltage applied between electrodes. The inversion cycle is one frame or a sub-frame obtained by uniformly dividing one frame.
When AC drive is performed to invert the polarity for each frame or sub-frame (to be referred to as frame inversion hereinafter), a pair of polarity-inverted displays need to be the same picture considering the purpose of avoiding the DC offset every pixel. However, even if signals are identical, the display sensitivity or non-linear characteristic to the drive voltage is not always the same or symmetrical between a case where the device is driven by a positive voltage and a case where it is driven by a negative voltage. The luminance slightly changes between these two cases, causing flicker.
To cancel the flicker, a direct viewing type LCD executes the above-described frame inversion, and at the same time, executes spatial polarity inversion, that is, pixel inversion to invert the polarity for each display element (pixel) (e.g., in a staggered pattern). Alternatively, the direct viewing type LCD executes line inversion to invert the polarity for each vertical or horizontal line.
This pixel inversion or line inversion causes the following problems in a method of performing projection display using a microdisplay, like a rear projector or front projector. That is, the electric field leaks between inverted areas (so-called disclination), adversely affecting the display picture. To prevent this, the microdisplay performs only temporal polarity inversion, that is, frame inversion without performing spatial polarity inversion, that is, pixel inversion or line inversion. Hence, the microdisplay suffers flicker.
To make flicker unobserved by the user, the microdisplay doubles the flicker frequency by doubling the refresh rate of the display. For example, when the frame frequency of an original picture to be displayed is 60 Hz, the refresh rate of the display device is set to 120 Hz to set the flicker frequency to 120 Hz. The 120-Hz flicker greatly exceeds the limit of the frequency recognizable by human eyes, so the problem of flicker is solved.
If the speed of the display device is doubled and 120-Hz frame inversion (to be referred to as double speed-frame inversion hereinafter) is done in order to practice the “sub-frame display method based on spatial frequency separation”, the following problem arises. That is, in double speed-frame inversion, a picture when driving the liquid crystal panel by a positive voltage and that when driving the panel by a negative voltage are different from each other. Thus, the DC balance in driving distorts in each pixel. In a liquid crystal microdisplay or the like, electrical charges are accumulated in each display element, and the residue image of a preceding picture appears.
To solve this problem, the polarity may also be inverted in a longer cycle separately from double speed-frame inversion. If the polarity is inverted in a long cycle, changes of the luminance and color stand out, degrading the image quality.